Effects of the Ce and Cr Contents in Fe–Ce–Cr Ferrite Spinels on the High-Temperature Water–Gas Shift Reaction

A series of Fe–Ce–Cr ternary oxide catalysts with different Fe/Ce/Cr atomic ratios have been synthesized using a coprecipitation method. The effects of the contents of both Ce and Cr in the Fe–Ce–Cr catalyst on the high-temperature water–gas shift (HT-WGS) reaction were investigated. Among the Fe–Ce–Cr catalyst with different atomic concentrations of Ce and Cr, the catalyst with a 10:1:1 atomic ratio showed the best performance in terms of HT-WGS activity. We found a strong synergistic effect between the dopants (Ce and Cr) and Fe that contributes the highest lattice strain and lowest crystallite size in Fe–Ce–Cr with an atomic ratio of 10:1:1 compared to other compositions. The Fe/Ce/Cr atomic ratio of 10:1:1 generated the highest lattice disorder in the Fe–Ce–Cr catalyst, as determined by Raman analysis. Our H 2 TPR results confirmed that the reduction temperature of hematite (Fe 2 O 3 ) to magnetite (Fe 3 O 4 ) increased upon the addition of Cr. Conversely, the incorporation of Ce significantly decreased the reduction temperature for hematite-to-magnetite conversion until the Fe/Ce atomic ratio reached 10:1. The XPS results demonstrated that only Ce acts as a chemical advocate for iron oxide in the HT-WGS by facilitating the surface Fe 2+ /Fe 3+ redox cycle through its Ce 3+ /Ce 4+ redox couple. However, Cr does not chemically support the iron oxide catalyst because of its single 3+ oxidation state. The Fe 2+ /Fe 3+ redox cycle was greatly facilitated over the surface of the catalyst at an Fe/Ce/Cr atomic ratio of 10:1:1 compared to the other investigated samples with different Fe/Ce and Fe/Cr compositions. The better HT-WGS performance of the Fe–Ce–Cr (10:1:1) catalyst compared to all of the other catalysts can be mainly ascribed to its high lattice strain or disorder and excellent surface redox properties, linked to the Fe 2+ /Fe 3+ and Ce 3+ /Ce 4+ redox pairs.